Sensory responses of patients suffering from diseases such as leprosy neuritis (associated with Hansen's Disease), and similar diseases, have been tested in various ways. For example, in making motor response tests, measure of strength of pinch "pinchometers" and grasp "dynamometers", as well as "Voluntary Muscle Testers" have been used. Another previously employed method of assessing results of treatment of leprosy neuritis has been by testing sensory response by subjecting a skin area of a patient to contact by a cotton tip, a feather, a pin, a hair, or a nylon filament.
A typical well known prior method of making detailed sensory response tests has been by using a Weinstein-Semmes nylon filament pressure aesthesiometer, shown for example in FIG. 1. This has become a fairly popular sensory testing tool because of its mechanical simplicity and because it can be clinically standardized for use in roughly assessing nerve damage changes. However, it is subject to a number of serious disadvantages.
For testing over a reasonable range of sensory conditions, a relatively large number of suitably calibrated Weinstein-Semmes pressure aesthesiometers are required. For example, to cover a range of sensed stress values (gm/sq.mm) from about 20 gm/sq.mm to 170 gm/sq.mm, a typical set would consist of 11 aesthesiometers, each being calibrated to allow its filament to buckle at a designated value of stress along said range. The manufacturer provides markings M on the handle of the aesthesiometer to indicate a designated function of the measured buckling force F, wherein EQU M=log (10.times.F)
The following table shows the force measurements, stress and M calculations for a typical set of Weinstein-Semmes pressure aesthesiometers:
TABLE I __________________________________________________________________________ Manufacturers M,Calculated Measured Marking from Measured Measured Force Area Stress M Force F(gm) A(sq.mm) S = gm/sq.mm __________________________________________________________________________ 6.10 5.94 86.5 0.51 171 5.88 5.86 73.2 0.42 175 5.46 5.35 22.3 0.27 82.0 5.18 5.27 18.6 0.22 84.9 5.07 5.23 17.0 0.18 94.9 4.93 5.03 10.6 0.14 76.1 4.74 4.50 3.14 0.081 38.9 4.56 4.45 2.81 0.077 36.6 4.31 4.27 1.85 0.063 29.5 4.17 4.20 1.58 0.047 33.7 4.08 3.99 0.977 0.041 23.9 __________________________________________________________________________
The above data appears in Levin, S. et al, "Von Frey's Method of Measuring Pressure Sensibility in the Hand: An Engineering Analysis of the Weinstein-Semmes Pressure Aesthesiometer", Journal of Hand Surgery, Vol. 3, No. 3, May 1978, pp 211-216.
The term "stress" is defined as a function of the resultant internal force (reaction) that resists changes in the size or shape of a body acted on by external forces; "stress" in the nylon filament is the ratio of applied load to the corresponding cross-sectional area. Therefore, the "stress" will be the same as the pressure on the surface of the skin, wherein the contact area is equal to the cross-sectional area of the filament.
It will be noted that in using a set of Weinstein-Semmes aesthesiometers, a number of aesthesiometers must be used in order to locate the one which buckles at the minimum stress value at which the patient senses the applied pressure. This may require the handling of a substantial number of aesthesiometers before the appropriate aesthesiometer is located.
It was found that in a typical set of manufactured Weinstein-Semmes aesthesiometers the shapes of the ends of the nylon filaments were not uniform, some being flared, some being cut at a slant, producing a sharp point, and some having frayed ends. A frayed end has more area in contact with the skin than the area corresponding to the measured diameter of the associated filament. A sharp point on the end of a filament would elicit a response like that of a pinprick, rather than that obtained from contact with a flat filament face.
Thus, with a set of Weinstein-Semmes aesthesiometers, the ends of the filaments would need to be uniformly cut cleanly with a flat face, for valid measurements for stress computation and for sensory response characteristics consistent with actual filament cross-sectional area.
Therefore, there is a definite need for an improved aesthesiometer which avoids the above-described disadvantages and inconveniences inherent in the previously used devices.